The present invention relates to a dual site catalyst comprising a chromium oxide catalyst with a transition metal compound and a catalyst activator, and a method for the preparation of this catalyst. The catalyst is particularly suitable for producing polyethylenes, homopolymers and copolymers, having a bimodal or broad molecular weight distribution.
Linear polyethylenes, such as high density polyethylene (HDPE) and linear low density polyethylene (LLDPE), can be endowed with specific properties through a suitable choice of polymerization conditions and polymerisation catalysts, of which chromium oxide and metallocene catalysts are among the commercially most important. Chromium oxide catalysts are characterized in that their active sites have different ratios between propagation and chain transfer. Moreover, they tend to produce short polymer chains and incorporate comonomer units at a high frequency. As a result, the obtained polymer will have possible comonomers and side chains unevenly distributed between the macromolecules. Consequently, the produced polymers will have a broad molecular weight distribution, and these polymers will have good processability properties. The metallocene catalysts, often called single site catalysts (SSC) because all their active sites are equal, produces polymers having a narrow molecular weight distribution and an even distribution of possible comonomers and side chains along the polymer backbone. These polymers have desired properties like high impact strength, good transparency, and high hot tack. The main disadvantage is their poor processability due to to the narrow molecular weight distribution. A combination of the properties of chromium oxide and metallocene polymers would be highly desireable for many end uses, in particular will polyethylenes having a controlled broad or bimodal molecular weight distribution be advantageous in polymer processing by extrusion. Such polyethylenes have traditionally been produced by consecutive polymerisations at different conditions in two reactors in a series. Another approach has been to perform the polymerisations in the presence of a modified catalyst, including the use of combinations of different catalysts. However, when combining different catalysts, such as chromium and metallocene containing catalysts, they have a tendency to inactivate each other.
U.S. Pat. No. 3,378,536 discloses a process for the polymerisation of ethylene by the use of a two-component catalyst system consisting of (a) chromium deposited on e.g. silica, where the chromium is activated in an oxygen-containing gas at a high temperature, and then reduced with CO; and (b) chromium or vanadium arene where the arene is an aromatic, optionally substituted, C6 ring. The two catalyst components are preferably fed separately to the polymerisation reactor.
EP 088 562 discloses a modified polymerisation catalyst comprising a silica support with deposited chromium. Following oxidation in dry air, the chromium is modified by being contacted with a transition metal compound containing a xcfx80-bonded ligand. The transition metal is Ti, V or Cr, preferably Ti. Preferably, the ligand is an unsaturated carbocyclic or heterocyclic ring system containing 6 delocalized xcfx80-electrons, for example cyclopentadienyl. The disclosed catalyst is not particulate. Only the use of bis-toluene titanium is exemplified, and the obtained polyethylenes have a substantial degree of branching and a medium to broad molecular weight distribution.
U.S. Pat Nos. 5,330,950 and 5,408,015 relate to ethylene polymers having broad molecular weight distributions, obtained by the use of a catalytic mixture including a MgO-supported Ziegler catalyst and a chromium oxide catalyst.
U.S. Pat. No. 5,399,622 discloses a process for the polymerisation of ethylene. The polymerisation is started with a chromium oxide catalyst to obtain granules of low density polyethylene. The polymerisation is continued by adding a cocatalyst and an yttrium-containing catalyst of formula (Cp2YXx)yMzLn, wherein Cp is an optionally substituted cyclopentadienyl, X is a halide, M is an alkali metal and L is an electron donor ligand, to obtain an outer shell of high density polyethylene on said granules. The disclosed catalyst is not particulate.
EP 339571 discloses a process for producing polyethylenes having a broad molecular weight distribution by the use of a catalyst system consisting of: a catalyst component (A) comprising a silica support onto which there is deposited a titanium or chromium compound; a catalyst component (B) comprising a transition metal compound with a ligand having conjugated xcfx80-electrons e.g. Cp2ZrCl2; and a catalyst component (C) which is an aluminoxan, e.g. MAO. The MAO and the metallocene are not impregnated into the catalyst support and therefore the catalyst will hardly be a true dual site catalyst. It would be expected that the chromium oxide catalyst makes one type of particle and the metallocene another type of particle, optionally partly attached to each other, but anyhow making a very inhomogeneous powder. The two types of catalyst sites or precursor sites are not directly exposed to each other. There is no mention of any prereduction of Cr6+ to Cr2+. The polymerisations will result in polymers having an undesireable amount of low molecular weight polymers.
EP-A-0 287 666 discloses a process for the polymerisation of olefins in the presence of a catalyst composed of (A) a metallocene compound, e.g. CP2ZrCl2, optionally supported on silica, (B) an aluminoxane, and (C) an organoaluminium compound, e.g. triisobutyl aluminium. The obtained polymers have a narrow molecular weight distribution.
WO 96/14154 discloses a dual site catalyst comprising a calcined silica support impregnated with dibutylmagnesium, 1-butanol and TiCl4, and also impregnated with MAO and (BuCp)2ZrCl2. This catalyst, which contains both a non-metallocene and a metallocene transition metal component, is used in the polymerisation of ethylene in one single reactor to obtain polyethylenes having a bimodal molecular weight distribution.
The object of the present invention is to provide a dual site catalyst that may be used in a single polymerisation reactor to obtain ethylene polymers having a controlled bimodal or broad molecular weight distribution. It has now surprisingly been found a versatile dual site catalyst that combines the features of both prior art chromium oxide and metallocene catalysts. A remarkable advantage is that this catalyst produces chromium and SSC polyethylenes simultaneously in one and the same polymer particle. The polymer particles as polymerised will when processed give a homogeneous polymer melt. With this novel dual site catalyst polyethylene resins for a broad application area can be produced. The resins have particularly good extrusion properties and are especially well suited for processing by film blowing and blow moulding.
The present invention thus provides a particulate, modified chromium oxide catalyst for the polymerisation of ethylene or ethylene with xcex1-olefins, comprising:
a) a chromium-oxide catalyst comprising a chromium oxide combined with an inorganic support,
b) a transition metal compound comprising at least one cyclopentadienylic ring bonded to said transition metal, which cyclopentadienylic ring may contain hetero atoms, be unsubstituted or substituted, bonded to the transition metal through a bridge, optionally annealed to other ring structures, and if two cyclopentadienyl rings are present they may be bonded to each other through a bridge, and
c) a catalyst activator.
A method for the preparation of a catalyst for the polymerisation of ethylene, or ethylene with xcex1-olefins, comprises the steps of:
a) subjecting a chromium oxide catalyst precursor, comprising a chromium oxide combined with an inorganic support, to oxidising conditions to obtain the chromium in an oxidised state, and
b) impregnating the obtained chromium catalyst with a catalyst activator and with a transition metal compound comprising at least one cyclopentadienylic ring bonded to the transition metal and at least one ligand selected from the group comprising alkoxy, amido and hydrocarbyl radicals, a halogen and a hydride, bonded to the transition metal, which cyclopentadienylic ring may contain hetero atoms, be unsubstituted or substituted, be bonded to the transition metal through a bridge, and optionally annealed to other ring structures, and if two cyclopentadienyl rings are present they may be bonded to each other through a bridge, and
c) subjecting the thus obtained particulate catalyst to drying conditions.
In a preferred embodiment of the present invention the oxidised catalyst obtained in step a) above further is subjected to reducing conditions to obtain a major part of the chromium in a divalent oxidation state before being subjected to the impregnation step b).
The catalyst defined above is used in the homopolymerisation of ethylene or copolymerisation of ethylene with xcex1-olefins.